Building structures must be designed to safely withstand forces that may be applied thereto. As construction techniques improve, buildings are more capable of resisting loads that are applied thereto. Examples of loads that may be applied to buildings are those that result from earthquakes and windstorms. These forces may resolve within a structure as tension, compression, shear, torsion, or bending forces. Of the forces produced by such events on a building, horizontal (or shear) loads are significant. These horizontal forces attempt to shear (slide) the building off its foundation. Additionally, horizontal forces that develop in an upper story of a multiple story structure are transmitted to the lowest story primarily as in-plane shear loads on the lower story walls. In conjunction with shear forces, “uplift” or “overturning” forces also result on the structure. These uplift/overturning forces, generated in reaction to the moment of the shear force, attempt to lift and rotate the walls of the structure about a lower corner of the wall. In fabricating the structure, the structure must be designed with sufficient “shear resistance” so that the structure does not sustain excessive non-structural and/or structural damage or collapse due to applied forces, potentially resulting in extensive economic cost, serious injury or loss of life. Shear resistance can be further defined as the ability of a structure to absorb, dissipate, and transfer forces. To address the need to build a structure having sufficient strength, uniform building codes (“UBC's”) provide required building practices wherein the prescribed goal is life safety, but not necessarily to retain the building as habitable after a natural disaster.
Damage caused by forces resulting from seismic and hurricane events has exposed the need for improved force-resisting structures and/or structural elements for both new building structures and for retrofit into existing building structures.
Prior to the creation of the UBC's, early buildings were constructed having little or no capability to resist shear forces, uplift from foundations, and other loads. Walls of the structure were generally constructed only of vertical frame members with horizontal planks nailed across them. Later improvements included the use of diagonal wood braces, or diagonal sub-planking in the walls, with either shingles or some other outer layer to exclude weather and provide a finished exterior. However, as understanding of building performance in earthquakes and hurricanes continues to improve, the necessity for better structural properties has become more apparent and is being mandated by the UBC.
In general construction, the most common way of producing a shear wall is to use plywood sheathing attached to a plurality of vertical 2×4 or 2×6 inch wooden or metal framing members. The plywood sheathing is attached to the framing members with closely spaced nails/screws on the edges of the plywood panel. The use of the plywood sheathing and specified fastening patterns that are incorporated into all modern building codes has proven to be a very successful method of producing a wall having shear resistance. Analysis of damage caused in recent earthquakes, such as the 1994 Northridge earthquake in California, illustrated that in some cases, buildings built to the standards specified in the California UBC survived rather well. However, there were a substantial number of structural failures generally associated with openings formed in shear walls and stress concentrations on steel-frame building connections. Although, a building may remain standing after an earthquake, it still may be rendered uninhabitable due to non-structural and/or structural damage.
Problems caused by openings are twofold: stiffness reduction and stress concentrations. First, openings dramatically reduce the shear stiffness of the wall. For example, even comparatively small window openings will reduce the shear stiffness sufficiently that the wall can no longer be considered a continuous shear wall, thereby increasing the effective aspect ratio of the wall, wherein the aspect ratio is defined as the ratio of the height of the wall H to the width of the wall W. When the aspect ratio of the wall is increased, the overturning forces on the wall for the constant overturning moment (where the moment is determined by story height and shear force only) become higher and more localized.
Referring now to FIG. 1, there is illustrated an exemplary embodiment of an isolated shear wall 10 illustrating the balance of forces applied thereto. The force F is the shear force carried by the shear wall at the top edge due to a loading event such as an earthquake. The force must be reacted in shear at the foundation, shown by the opposing force F at the bottom. The moment of F relative to the foundation, equal to F multiplied by the story height H, must be reacted by foundation vertical or overturning forces A1, A2 (shown as discrete, but may be distributed near the corners). The force A2 is particularly troublesome, as it is tensile against the foundation, and is equal to (H/W)×F. In a case where there are adjacent additional structures, some of the overturning moment may be carried by shear on the sides of the shear wall 10, but eventually the entire overturning moment must be reacted at the foundation by vertical forces, and those forces are proportional to the panel aspect ratio H/W.
Referring now to FIG. 2A, there is shown an exemplary embodiment of a shear wall 10 wherein an opening b has been formed within the shear wall. As shown in FIG. 2A, the opening creates a discontinuity in the force transmitting characteristics of the shear wall, wherein forces that are normally carried across the entire wall width W now must be carried across the reduced width W′. The reduced width is less stiff and less strong, and the opening corners also introduce panel stress concentrations that did not previously exist. The corners A tend to crack open, and the corners B tend to crush and buckle closed, under the direction of force F′ shown, as FIG. 2B shows. Therefore, the load carrying stiffness and overall strength of this shear wall is substantially reduced. In addition, if adjacent structures exist, they will be caused to carry more forces because this panel is less stiff and as a result takes up a smaller proportion of the forces.
To address the weakness created in shear walls due to openings formed therein, there have been recent changes in the UBC. The recent changes to the UBC have halved the maximum aspect ratio of shear walls and shear wall segments so that the minimum width of an 8 ft high shear wall has been increased from 2 ft. to 4 ft, for a maximum aspect ratio of two.
Another problematic variable in the construction of a building is the variations in construction quality, foundation quality, and soil variability. Following the 1994 Northridge earthquake, it was discovered that a large percentage of building failures occurred as a result of poor field construction practice. One study indicated that one third of the seismic safety items installed were missing and/or improperly installed or poorly implemented in over 40% of the structures surveyed.
Further still, it is important that structural elements within the building structure have generally similar strength and stiffness properties in order to share the applied loads. If every structural element does not work together, this may lead to excessive damage or failure of a structural element due to force over-loading of the structural element, as opposed to load sharing. There may be locations within a building structure wherein walls having different stiffness/strength are joined together. For example, a structure may be built with a concrete retaining wall, wherein timber-framed shear walls may be joined to the poured concrete retaining wall. Many times, during seismic events the connection point of the two walls having different stiffness will separate due to the difference in stiffness of the walls in relation to the movement of the wall in response to the seismic event. In addition, irregular placement of structural elements with varying stiffness/strength characteristics can result in twisting of the structure leading to additional torsional stresses and other stress amplifications. Thus, there is a need for a device that will transmit forces and dissipate and absorb energy across discontinuous structural elements.
In addition to that above, another aspect to be considered is the manner in which the UBC is interpreted by local building inspectors. Often, building inspectors will make highly restrictive interpretations of the building codes in an effort to promote increased safety in building practices.
There have been numerous attempts to address increasing the shear resistance of a structure where the structure includes a number of discontinuities/openings formed in shear wall(s). One of the most common methods of addressing the need to increase the shear resistance of a structure has been to include a moment frame in the design of the structure, whereby steel beams are rigidly connected together such that any force applied to the structure will be carried through the moment frame. A moment frame is typically embodied as a large heavy steel structure designed to transmit shear forces of the structure into the foundation or into special footings formed in the foundation, via bending (or moment) resistance of large steel members. However, a moment frame must be specifically engineered for each application, thus adding significant cost and complexity to the structure. In residential construction, even a modest opening in a shear wall can require 6″ or 8″ steel girders weighing hundreds of pounds and the attendant foundation reinforcement required to absorb the loads transmitted thereto by the moment frame. The architect/builder must also account for shipping and handling costs associated with the installation of these heavy steel beams on the building site. Further still, the use of a moment frame causes significant problems with the insulating properties of the building, as the metal beams act to conduct heat through the walls of the structure to the interior of the structure, thus causing degradation of insulation properties.
Although moment frames appear to be a solution, albeit inefficient, to increase the shear resistance of a structure, there are still shortcomings of the popular field welded-field bolted beam-to-column moment frame connection. Observation of damage sustained in buildings during the 1994 Northridge earthquake showed that, at many sites, brittle fractures occurred within the connections at very low levels of loading, even while the structure itself remained essentially elastic (Federal Emergency Management Administration Report 350). This type of connection is now not to be used in the construction of new seismic moment frames. For example, tests conducted by the Seismic Structural Design Associates, Inc. (SSDA) have shown large stress and strain gradients in moment frame joints/connections that exacerbate fracture. To address these large concentrations of stress in the corners, there has been much work attempting to improve the ability of the corners of a moment frame to resist loads. One such improvement to a corner connection is embodied in U.S. Pat. No. 6,237,303.
Another approach to structural reinforcement is to utilize a pre-built shear wall such as the Simpson StrongWall®. The StrongWall® is a pre-built shear wall that may be integrated into a building structure. The StrongWall® is constructed of standard framing materials and metal connectors. The StrongWall® further includes a plurality of devices configured to anchor the StrongWall® to a building foundation. The StrongWall® must be connected to the framing of the structure as well as to the foundation. Because the StrongWall® must be connected to the structure's foundation, this requires special work on the foundation prior to installation, thus rendering retrofit application of the StrongWall® not cost effective. In addition, the StrongWall® is delivered to a job site as a pre-built panel, thus the architect/builder must account for shipping and handling costs associated with the installation of these heavy panels on the building site.
Shortcomings of both moment frames and StrongWalls® are that both devices do not attempt to match the shear stiffness and strength characteristics of the surrounding structure. Instead, each device is designed without regard for the structure it will be used within, and is generally designed to carry the entire shear load of a wall or wall segment. As described above, a moment frame is typically constructed of steel beams, wherein the beams are rigidly connected together such that any force applied to the structure will be carried through the moment frame and into the foundation. The StrongWall® is designed in a similar manner, wherein the StrongWall® attempts to be stronger than the surrounding structure. Moment frames and larger StrongWalls®, due to their size and weight, can be difficult to move around the job site and install without the use of costly heavy equipment. Both the moment frame and the StrongWall® significantly increase the overall cost of the structure. Therefore there is a need for a lightweight device that may be installed within or about openings of a structure to maintain the properties of that structure as a generally continuous element.
While the two devices described above may be readily utilized in new construction there is still a need for devices that may be utilized during structural retrofits, seismic or hurricane upgrades, and/or remodels. For example, a homeowner may cut an opening in a shear wall to place a new window or doorway. Many times, these home retrofits are done without any consideration to shear strength of the wall or obtaining a permit. Thus, when the homeowner wishes to sell their house that includes these “improvements”, many times their homes will not meet code and cannot be sold as is. What is therefore needed is a device that can be readily adapted to retrofits to maintain the properties of the structure as a generally continuous element after an opening has been formed in the shear wall. There is also a need for an easily manufactured, lighter, less complicated, more versatile, adjustable, easier to install device for new construction.